This invention relates to an audio system.
As shown in FIG. 7, a conventional stereo system includes at least two speaker units 33 and 34 containing speakers 31 and 32 and disposed horizontally separated from each other by a distance D such that a listener (not shown) will listen to the sounds from the speakers 31 and 32 by facing the speaker units 33 and 34 at a position on a line L6 which perpendicularly bisects the line segment L5 of length D connecting the front surfaces thereof. Since good acoustic effects are obtainable from the individual speakers 31 and 32 if the listener is positioned within the fan-shaped areas around their central axes L7 and L8, each subtending a certain angle a, good stereophonic effect is generally obtainable in the area A where the aforementioned two fan-shaped areas overlap with each other, or the area shaded in FIG. 7. If the listener is on the bisecting line L6 at a position indicated by P in FIG. 7, the minimum value of the distance F between the point P and the line segment L5 at which a good stereophonic effect may be expected depends on the transverse separation distance D between the speaker units 33 and 34 and the angle .alpha. determined by the speakers 31 and 32. The angle .alpha. depends partially on the speakers' diameter and may be about 30.degree. if the diameter is 12 cm.
With a stereo system of the type described above, the area within which the sounds from the speakers 31 and 32 can be received well is extremely limited. On the other hand, there are frequently situations wherein it is desired to expand such preferred area, for example, such that listeners can enjoy good stereophonic sound effects anywhere within a certain radius from a single source as illustrated in FIG. 8 wherein numeral 41 indicates a single speaker having a central axis L8 defining the X axis of a coordinate system therearound, the Y axis being perpendicular thereto and passing through the position of the speaker 41 and letter B indicating a planar area defined by the X and Y axes. In other words, speakers which are non-directional within a plane are desired and if two or more of such speakers are appropriately positioned within a room, for example, persons can hear the sounds from these speakers distinctly from any position inside the room and enjoy different acoustical effects than those from a conventional stereo system. Nowadays, there are increased demands for such an audio system that can create a so-called "free acoustic space".
If the central axis of a single speaker is frontally directed to a listener, however, this does not amount to providing a non-directional speaker. In the past, there was a proposal for so-called non-directional spherical speakers having a plurality of speakers arranged inside a ball-shaped speaker unit such that sounds will be propagated in all directions from a center at the speaker unit. This proposal, however, was not well accepted because many speakers are required and this increases the overall production cost and also because such a system must be disposed and supported correctly for a good result and hence cannot be used easily.